Motionless mixers are static mixing devices generally used to intermix two viscous fluids. For example, one may wish to mix a thermoset, which consist of a resin and hardner, e.g. epoxy. This can be done by simultaneously passing both the hardener and resin, in their liquid forms, into a conduit of a motionless mixer containing a multiplicity of baffles. As the fluids travel down through the bore of the conduit they are intermixed in stages corresponding to each baffle of the mixer.
In the past, motionless mixers have employed multiple metallic baffle elements. These baffles were easily made but complicated to assemble in series for use in a static mixer.
Today, conventional motionless mixers are more typically manufactured from plastic by injecting molding, thereby considerably reducing production costs when made on a large scale basis. Representative examples of such motionless mixers are disclosed in U.S. Pat. Nos. 3,286,992 and 3,953,002 and 3,635,444. The plastic mixers are generally comprised of alternately right- and left-handed helically-curved baffles which are either individually disposed within a bore or are adjacently combined during manufacture to form a single unit insert which is disposed within a bore.
The leading edges employed on these plastic baffles vary in design. In one known design, the two major opposing curved surfaces defining the baffle terminate in and are joined by a planar surface extending from, perpendicular to and lying in a plane normal to the central longitudinal axis of each baffle. A problem encountered in using plastic baffles of this type is the decreased efficiency of the mixing process. Viscous materials such as thermoplastics, resins and various other polymers tend to accumulate and build up on the flat surfaces as they pass over the baffles, thus decreasing the efficiency of the mixing process and oftentimes completely blocking the mixer and stopping fluid flow. The flat leading surfaces also cause a substantial reduction in flow cross-section at the intersection of baffle elements, for example at the intersection of elements of 0.5 inches diameter with a 0.125 inch baffle thickness the available flow area is only 40% of the overall cross-sectional area. This reduction in flow cross-section results in substantial fluid pressure drop.
Each of the baffles of another known arrangement include a knife-like edge at one end (the upstream end) formed by tapering the two curved major opposing surfaces of the baffle towards one another. Motionless mixers of this type, present problems in manufacture. Injection molding a baffle having a knife-like edge formed by tapering a pair of major opposing curved surfaces of the baffle towards one another would be both extremely difficult and costly. This is because more than two mold pieces would be required in order to avoid undercuts. This makes the baffle substantially more costly and difficult to produce.
Generally, if not without exception, the baffles forming motionless mixers in the prior art have had a cross-section which is rectangular so that the passages defined by the baffles within its encompassing tube are substantially semi-circular (i.e. a section defined by a portion of a circle subtending an angle approaching 180.degree., the ends of which are joined by a straight line). As the fluids spiral through these passages the fluid, due to the helical flow pattern, rotates about the center of each passage (i.e. inverts). Optimum performance is achieved only if the fluids rotate 180.degree.. In the past the flat sheet used to make elements, which yield two opposing semicircular or "half moon" cross sections. With this profile the length/diameter (L/D) of the element must be between 1.0 and 1.5 for optimum performance. This L/D ratio produces 180.degree. rotation of the fluid.
The semicircular subchannels resulting from the use of flat elements with rectangular cross sections is acceptable with L/D of greater than 1. However when the L/D ratio is reduced to 0.8:1 or 0.7:1 the amount of rotation decreases. This is particularly true in the corners where the baffles meet the wall where the velocity gradients are near zero. The fluid near the corners is simply dragged along the edge of the element and never mixes with the fluids in the center.
Accordingly, it is an object of the present invention to provide an improved static mixing device for intermixing a plurality of fluids that has a shorter length than previously possible.
More specifically, it is an object to provide a static mixing device of relatively economic construction and improved intermixing efficiency.